1. Technical Field
This invention relates to the field of cathodic protection for reinforced concrete. The present invention is particularly applicable to a sacrificial cathodic protection system.
2. Description of the Prior Art
Cathodic protection is a proven means of stopping the corrosion of steel in waters, soils and moist, salty reinforced and prestressed concrete structures both above and below water and soil.
Work on adapting sacrificial anodes to reinforced and prestressed concrete structures began in the mid-1970's. Although such anodes could be made to work, problems with low power output and accommodation of the anode corrosion products prevented widespread use of the sacrificial anode technology.
The alternative to a sacrificial anode system is an impressed current cathodic protection system in which power from an outside source is used in concert with low corrosion rate anodes. This alternative does overcome the difficulties of low power output and accommodation of anode corrosion products. Many impressed current systems have been installed. However, they are costly and complicated.
Present sacrificial anode systems will work in hot, moist environments, such as the Florida Keys and similar coastal areas. However, they are too low power for above-water and soil concrete structures in the central and northern portions of the United States. They are also too low power to halt corrosion on structures in hot, moist climates wherein the steel is corroding at a high rate.
The causes of low power output are a low voltage or potential difference between the sacrificial anode and the corroding steel in salty concrete, generally less than one volt, and oftentimes less than 0.6 volts. In addition, concrete, even when wet, has a higher resistivity (resistance per unit area) than most wet soils and natural waters, up to 100,000 or more ohms-cm, which creates a circuit resistance of hundreds or thousands of ohms. Dividing the voltage, e.g., 0.6 to one volt, by the resistance, e.g., hundreds or thousands of ohms, resulting in a low current output.
Temperature greatly affects concrete resistivity. The lower the temperature, the higher the resistivity and thus the lower the current output.
The resistance is increased if the concrete layer immediately beneath the sacrificial anode dries out.
Additionally, sacrificial systems have the shortcoming of a finite life which often is less than the life of the structure involved. If the sacrificial systems are embedded in concrete or otherwise attached in a high cost manner, they are expensive to repair and/or replace.